The use of nuclear magnetic resonance phenomena for the purpose of producing images is becoming increasingly well known. This is particularly true in the medical and diagnostic arts where magnetic resonance imaging (MRI) systems have become a commonplace and often indispensable tool for the diagnosis of many conditions.
A typical MRI system, of the type used for medical diagnosis, includes four major components. The first of these components is a large fixed-pole magnet. The fixed-pole magnet is formed to establish a channel dimensioned to receive a patient's entire body. The fixed-pole magnet may be of the permanent or electromagnetic type. Increasingly, however, the fixed-pole magnet is constructed as a cryogenic superconducting magnet, due largely to the ability of such magnets to provide a strong, stable magnetic field without the great mass associated with permanent magnets or the high power consumption associated with electromagnets. Functionally, the fixed-pole magnet operates to establish a strong and highly homogeneous static magnetic field within the channel established by the magnet.
The second major component of a typical MRI system is a system of gradient and radio frequency coils. Functionally, these coils, which may be implemented using a wide range of differing designs and types, operate to direct weak magnetic fields into the channel established by the fixed-pole magnet. The coils are positioned so that the weak magnetic fields are directed along a three-axis coordinate system. Additionally, the coils include a control mechanism that allows the fields to be alternately enabled and disabled.
The third and fourth major components found in a typical MRI system are a radio sensing system and a computer imaging system. The radio sensing system is, in simple terms, an antenna, which operates to collect radio frequency energy emitted by atomic nuclei under conditions created by the combined magnetic fields present in the MRI system. The computer imaging system is connected to the radio sensing system and performs various convolutions on the signals gathered by the radio sensing system to produce visual images corresponding to those signals.
In the operation of a typical MRI system, a patient, or other object, is positioned in the channel established by the fixed-pole magnet. Once positioned, the static magnetic field produced by the fixed-pole magnet interacts with some of the atomic nuclei contained within the tissues of the patient. As a result of the interaction caused by the static magnetic field, these atomic nuclei will tend to become aligned with the static magnetic field. The result is that the static magnetic field will tend to impart a net magnetic orientation to the atomic nuclei contained within the tissues of the patient.
While the patient is positioned within the channel established by the fixed-pole magnet, the gradient and radio frequency coils are selectively enabled and disabled to establish a series of weak magnetic fields within the channel. Not unexpectedly, the atomic nuclei which have become aligned by the static magnetic field become realigned under the influence of the weak magnetic fields. The alignment produced by the weak magnetic fields is, however, temporary and each atomic nuclei regains its normal orientation when the weak magnetic fields are disabled.
The return of the atomic nuclei to their normal orientation is known as spin relaxation and is accompanied by a release of radio energy from the atomic nuclei. The energy emitted during spin relaxation is known as a spin echo and is received by the radio sensing system providing the sensing system with information about the location of each nuclei that undergoes the process of relaxation. By alternately enabling and disabling weak magnetic fields, the location of the atomic nuclei may be accurately recorded by the radio sensing system. The information gathered by the radio sensing system is transmitted to the imaging computer system where the location of each atomic nuclei may be plotted in a three-dimensional image of the patient's tissues.
The basic MRI technology, as described above, has proven to be a highly effective tool for medical and other diagnostic purposes. In fact, the effectiveness of this technology has lead to the widespread use of MRI systems, making such systems somewhat commonplace in the medical field. Not unexpectedly, the success of MRI technology has also spurred numerous efforts to produce improvements to the basic MRI system. Many of these efforts have been directed at the production of MRI systems which produce high resolution images of large anatomical objects viewed.
One way of enhancing MRI image quality is the use of magnetic systems which produce highly uniform, or homogeneous, magnetic fields within the channel. Unfortunately, it has long been recognized that no magnetic field is perfectly homogeneous. Instead, the strength of the field produced by a magnet, or flux density B.sub.0, varies as a function of position in relation to the magnet's two poles. For example, if it is assumed that an axis Z passes through the north and south poles of a magnet, it will be the case that the flux density B.sub.z will reach a maximum value at locations on the axis Z which are immediately adjacent to the north or south poles. At the same time, B.sub.z will reach a minimum value at the point on the Z axis which is midway between the north and south poles. Mathematically, saying that B.sub.z reaches a minimum value at the point midway between the north and south poles is equivalent to saying that the first derivative of B.sub.z or ##EQU1## is zero at that point.
MRI manufactures have long appreciated that more extensive homogeneous magnetic fields may be produced by using larger magnets to increase the volume, or sweet spot, around the midpoint where ##EQU2## is zero, or nearly zero. At the same time, it has been recognized that even this objective also achieved by fields whose higher order derivatives of B.sub.z, such as ##EQU3## are also zero. For example, U.S. Pat. No. 5,400,786 which issued to Allis for an invention entitled "MRI Magnets" is directed at a specific arrangement of magnets and shims intended to produce a substantially homogeneous magnetic field. In any event, with an increase in the relative size of the homogenous field region, a larger anatomical volume of tissue may be imaged.
The present invention recognizes that large volumes of tissue do not always need to be imaged. Indeed, MRI systems may be constructed which are usefully directed at the production of relatively small images such as a small sample of the skin tissue. MRI systems of this type may be constructed by utilizing a static magnetic field which exhibits less extensive, homogeneity having a sweet spot where ##EQU4## is zero but wherein higher order derivatives, such as ##EQU5## are non-zero. Potentially, low-cost MRI systems of this type might be directed at imaging human skin, both for diagnostic as well as educational purposes.
In light of the above, it is an object of the present invention to provide an MRI system which is useful for the in vivo imaging of very small superficial volumes of tissue, such as skin samples. Another object of the present invention is to provide an MRI system whose imaging volume is small in comparison to the overall size of the system. Yet another object of the present invention is to provide an MRI system which is relatively simple to manufacture, easy to use, and comparatively cost effective.